USB is one type of connection to allow transfer of data between electrical systems or devices. The Universal Serial Bus Implementers Forum (USB-IF), in Portland, Oreg., U.S.A., manages and publishes the specifications for USB. Several specification revisions of USB have been published by USB-IF. At the time of this writing, USB 2.0 published in Apr. 27, 2000 is the latest USB specification revision and is compatible with earlier USB specification revisions such as USB 1.0 and USB 1.1. In the description described herein, USB 2.0 refers to the USB 2.0 specification revision and all other earlier USB specification revisions published by the USB-IF.
USB 2.0 includes a suspend operation to allow a USB device to enter a suspend state. USB 2.0 requires a USB device to enter the suspend state after a specific time of idling or inactivity on the USB connection. USB 2.0 also includes a resume operation to allow a USB device to exit the suspend state. USB 2.0 requires the resume operation to last for some specific time.
The timing requirements for the suspend and resume operations as specified in USB 2.0 may limit the power management for some systems having USB. For other systems, especially for systems that draw power mainly from a battery, the timing requirements for the suspend and resume operations may be too large, causing USB as specified in USB 2.0 to be an unfavorable choice for connectivity in some of these systems.